The present invention relates to a method and a device for creating a symbol in a workpiece surface via stamping.
The creation of symbols, e.g. alphanumeric characters, in workpiece surfaces is used, e.g. in the automotive industry to provide workpieces such as chassis, engines, or the like with largely unadulterable markings in the form of labeling or other types of identification. The stamping tools used for this purpose are stamping needles in particular, which compose the symbols in a matrix form using a plurality of points (needle stamping), or as plain text, in that the stamping needle is pressed into the workpiece surface and then moved transversely thereto (scoring or plastic forming under compressive conditions).
In the latter two cases, in which the individual symbols are designed as continuous, essentially V-shaped score lines or grooves, it is increasingly required in order to obtain a highly legible, uniform typeface that the symbols have a continuously uniform scoring or groove depth, referred to hereinbelow as the stamping depth, of, e.g. 0.2 mm. Since it is practically impossible to perform a final visual inspection of the stamping depth, and given that the stamping devices that are typically used do not ensure that a specified stamping depth may actually be attained and adhered to, methods and devices of the general classes described initially are required, using which the stamping depth is determined automatically, and using which it may be automatically ensured that a preselected stamping depth is always attained.
Known methods and devices of the general classes described initially use fully optical means to determine the stamping depth (e.g. DE 199 30 272 A1, DE 10 2005 037 411 A1), which include sensors, e.g. that operate using laser light. The disadvantage is that a working step must be carried out after the stamping in order to measure the stamping depth that was actually attained, and these methods are susceptible to contamination, in particular due to the unavoidable ejection of material that is displaced during scoring. The stamping depths attained therefore do not conform to the specified setpoint values with an adequate level of reliability. It is also known to first determine the distance between the stamping needle and the workpiece surface using a capacitive proximity sensor, and to then move the stamping needle in the direction of the workpiece surface. This method does not ensure a high level of accuracy, either, because the workpiece may bend during the stamping procedure, e.g. if it is designed as a piece of sheet metal, which then results in a stamping depth that is shallower than the desired stamping depth.
Finally, it would be possible to inspect the stamped labels using a camera. As a result, however, it is typically possible only to determine whether the desired symbols are actually present. In addition, any contaminating particles (ejected material) that are present, changing light conditions, and different colors of the workpieces have an unfavorable effect on the measured results, which is why poor availability is attained using these methods and devices as well.